Labor unions long legendary for their powerful get-out-the-vote machines face an unprecedented test this year, as unfettered conservative groups spend record sums on campaign ads and newly minted ground operations.

Some GOP organizers argue that labor “bosses” still outspend them, based on estimates that unions dumped $450 million or even far more into the 2008 election. But that claim is losing credence in the first presidential election since the Supreme Court’s 2010 ruling that deregulated political spending.

GOP-friendly super PACs and nonprofit groups have already spent more than $472 million in this campaign, according to the Center for Responsive Politics, more than twice the $195 million or so that liberal groups, including unions, have spent.

From GOP-friendly super PACs such as American Crossroads to conservative nonprofits such as Americans for Prosperity, anti-labor groups are not just spending hundreds of millions of dollars. They’ve also set out to beat labor leaders at their own game, establishing field offices nationwide and deploying busloads of volunteers to meet face to face with voters in the campaign’s final weeks.

“We need to be the grass-roots movement that the left is,” said Ron Meyer, spokesman for American Majority Action, a conservative nonprofit that will spend $5 million or more exclusively on GOTV and field operations. In Wisconsin alone, American Majority Action has opened five field offices and plans to knock on 100,000 doors between now and Election Day.

Americans for Prosperity, which has received generous backing from billionaire industrialists Charles and David Koch, has full-time staff members in 32 states and has spent four years building a grass-roots network in Ohio, the group’s organizers say. AFP announced plans to mobilize staff and volunteers to call more than 8 million voters and knock on 100,000 doors by the end of this month.

Labor organizers know they’ve got competition. Union members canvassing neighborhoods are bumping into conservative door-knockers from groups such as AFP, the tea party organization True the Vote and Ralph Reed’s Faith and Freedom Coalition.

“We have respect for the political skill of people on the other side,” AFL-CIO spokesman Jeff Hauser said, adding that labor foes “are more committed to the ground than they once were, and we take them seriously.”

Conservatives largely run paid ground operations, Hauser noted, in contrast with union drives powered mostly by volunteers. The conservative movement “doesn’t have the membership base or the human energy, but they have a lot of money,” he added. “Money is an equalizer.”

Not that unions won’t play an influential — perhaps even decisive — role in this election, which by many estimates will hinge on which side most successfully gets its voters to the polls. Like corporations, unions also face fewer spending restrictions in the wake of Citizens United v. Federal Election Commission.

Unions may now spend more freely contacting non-union voters, as opposed to simply mobilizing their own members. From the AFL-CIO to the Service Employees International Union, from the National Education Association to the American Federation of State, County and Municipal Employees, unions have generously powered Democratic-friendly super PACs.

The four top super PACs backing Democrats — Priorities USA Action, Majority PAC, House Majority PAC and American Bridge 21st Century — have collected almost $20 million from labor unions this election cycle, according to the Center for Responsive Politics and Federal Election Commission reports. Last month alone, those four super PACs collected almost $6 million from a long list of unions.

The SEIU’s Committee on Political Education collected $20.3 million in 2011 from 300,000 members, a 30 percent increase over 2008, organizers say. The SEIU plans to mobilize thousands of volunteers in eight battleground states between now and Election Day, including knocking on 550,000 doors in Ohio alone. The AFL-CIO has mobilized 400,000 volunteers on the ground.

“We knew we were facing an opposition that was more funded than in a generation and that this election was going to come down to turnout,” SEIU International President Mary Kay Henry said. The 2.1 million-member SEIU ramped up its ground operations and launched them earlier because of “the flood of communication and money from the other side,” she added.

Big labor spending on campaigns tends to go unreported, conservatives said, because much of it takes the form of member-to-member contacts that are not publicly disclosed to the FEC. Last year, the Wall Street Journal combed through union expenditures reported to the Labor Department to conclude that such spending from 2005 to 2011 totaled $4.4 billion.

“Looking at the most recent elections, unions represented the largest-spending groups in campaigns,” American Crossroads spokesman Jonathan Collegio said.

Labor leaders dispute the Journal’s estimate, arguing that it counts all lobbying activities as election-related and double-counts expenditures. Even some conservatives acknowledge that Citizens United has changed the math for unions, already buffeted by declining membership and collective bargaining fights in states around the country.

The point isn’t how much unions spend, Meyer said, but how they spend it. On that score, he maintained, unions are still way ahead.

“As far as the ground game goes, we’re not competing with the left yet — we’re not even close,” Meyer said. But conservatives are quickly catching up.